Thread: rpm 100km?

I did a bit of juggling with the comparator

Ashcroft Transmissions - Ratio Calculator

and I think I have an answer:

1. Are you sure you have a 1:1.22 TCase?

2. If the 235/85's are worn, they could end up being a bit more like 31 inches

Because when you input a defender with stock GB/TC, 4.11 diffs and 31 inch tyres you get 5th a little over 3100 rpm. And, a 300tdi doing that will be at full tilt ie 15psi.

I own a Disco with 235/85's and on the highway it cruises along nice - but that's about it. With those tyres that's about the limit of power for a normal 300tdi.

Boost depends so much on road surface, whether you have a slight incline/decline, tail or headwind, etc.

On the same stretch of road my boost @ 100km/h has varied between 10.5 and 15psi, depending on whether I have a head or tailwind.

As langy said, tyre size can vary dramatically, even amongst new tyres of a given size between brands.
The only way to calculate RPM's correctly is run a tape around the circumference of a tyre at your standard pressure and calculate from there.

That set up seems to defeat it's purpose? The diff ratio change and transfer case gearing change are normally done for fitment of larger tyres.....

Running 235/85R16 tyres just doesn't make any sense to me. Unless you are already planning to go to a 285 tyre?




I run 235/85R16 on my 200tdi Defender, and it's got the Disco high range gears in the transfer case. The diff ratio's are stock standard. On slight uphill take-off's I need to slip the clutch a little especially when towing. On the highway it sits around mid revs quite comfortably, and will quite easily hold 4th gear at 100km/h if required due to a head-wind etc.


Cheers
Chris

1. Are you sure you have a 1:1.22 TCase?

well i did not personal do the change over but i have receipts describing the work and the old gear set . the old owner had the work dun.
i have had my doubts about it


2. If the 235/85's are worn, they could end up being a bit more like 31 inches
tires are at about 80%


That set up seems to defeat it's purpose?

i have a set of 285 75 16 sitting in the shed so i thought the diff gears would be needed to combat the high range and slight up in tire size.
and the low range is a bit lower.

thanks for the help
cheers wmd

what does your tacho read at idle?

that should tell you if the tacho is out.